Docs: updated "Upgrading" for docker driver

[till]

What this PR does / why we need it:

• I recently updated a couple of the loki docker plugins we use
• no update would work without restarting docker
• specifically, reading docker [service] logs $foo no longer worked
• restart of dockerd fixes it (no logs seem lost)

Here is a docker plugin inspect loki from one of the hosts pre-update:

[
    {
        "Config": {
            "Args": {
                "Description": "",
                "Name": "",
                "Settable": null,
                "Value": null
            },
            "Description": "Loki Logging Driver",
            "DockerVersion": "17.09.0-ce",
            "Documentation": "https://github.com/grafana/loki",
            "Entrypoint": [
                "/bin/docker-driver"
            ],
            "Env": [
                {
                    "Description": "Set log level to output for plugin logs",
                    "Name": "LOG_LEVEL",
                    "Settable": [
                        "value"
                    ],
                    "Value": "info"
                }
            ],
            "Interface": {
                "Socket": "loki.sock",
                "Types": [
                    "docker.logdriver/1.0"
                ]
            },
            "IpcHost": false,
            "Linux": {
                "AllowAllDevices": false,
                "Capabilities": null,
                "Devices": null
            },
            "Mounts": null,
            "Network": {
                "Type": "host"
            },
            "PidHost": false,
            "PropagatedMount": "",
            "User": {},
            "WorkDir": "",
            "rootfs": {
                "diff_ids": [
                    "sha256:6f56fdd1c41a57bc4631c3892095595851a49305ae7be76982d0c542e9089a2c"
                ],
                "type": "layers"
            }
        },
        "Enabled": true,
        "Id": "e907aee1219b835af554442134456ed8739e7a989ea19eac131b77126816c0d9",
        "Name": "loki:latest",
        "PluginReference": "docker.io/grafana/loki-docker-driver:latest",
        "Settings": {
            "Args": [],
            "Devices": [],
            "Env": [
                "LOG_LEVEL=info"
            ],
            "Mounts": []
        }
    }
]

Checklist

• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #2439 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2439      +/-   ##
==========================================
+ Coverage   62.70%   62.72%   +0.02%     
==========================================
  Files         162      162              
  Lines       13850    13850              
==========================================
+ Hits         8685     8688       +3     
+ Misses       4488     4487       -1     
+ Partials      677      675       -2     

Impacted Files	Coverage Δ
pkg/querier/queryrange/downstreamer.go	95.87% <0.00%> (-2.07%)	⬇️
pkg/promtail/targets/file/filetarget.go	69.64% <0.00%> (+1.78%)	⬆️
pkg/promtail/targets/file/tailer.go	76.13% <0.00%> (+2.27%)	⬆️

[till]

Maybe another thing, this is the error message, prior to restarting docker:

$ docker logs ffa478243888
error from daemon in stream: Error grabbing logs: error getting log reader: LogDriver.ReadLogs: logger does not exist for ffa478243888530bb9bf6d1d973d831f7b619056c275f016be226fb03801f158

[cyriltovena]

LGTM